For many commercially available CT scanners, the photons emitted from the X-ray source spread within a wide and continuous distribution of energy spectrum. As a poly-energetic X-ray beam passes through matter, lower-energy photons are preferentially absorbed compared to higher-energy photons; the beam gradually becomes harder as its mean energy increases. This phenomenon leads to various well-known beam-hardening (BH) artifacts, such as cupping and streaks, which affect the voxel values in the reconstructed image, and make the quantitative evaluation of attenuation properties challenging.
Ever since the first clinical CT scanner in 1967, numerous efforts have been made to address this challenge via poly-energetic reconstruction. These methods aim to incorporate the energy-dependent nonlinearity of the attenuation coefficient into the reconstruction process without a dramatic increase in computational complexity. Generally speaking, these methods can be divided into three categories.
The first category is filtered backprojection (FBP)-based linearization approaches, which aim to correct the raw measurements according to the water's or bone's X-ray attenuation characteristics prior to the FBP reconstruction. For example, a reconstruction technique (i.e., water correction) was previously developed to compensate for the water-related cupping artifacts. However, this technique is limited to soft tissue, and for inhomogeneous objects (especially in the presence of bone), BH artifacts are still significant. Later, a bone correction technique was developed for X-ray images. However, this technique requires a threshold-based segmentation process for the initial reconstructed images, which restricts its quantitative performance.
The second category is iterative-based base material approaches, which assume that the target volume consists of N known base materials, and use a linear combination of the known energy dependences to approximate the energy dependence of the attenuation coefficient in each voxel. Base material approaches are a more accurate alternative to the linearization approaches, but limited base materials have been incorporated due to expensive computation. For example, a segmentation-free displacement model was developed that accounts for soft tissue, bone, and their mixture by their density difference. This technique performs well for these base materials, but is less accurate for fat and breast tissue since their spectral properties deviate from those of the base materials.
In the third category, as an acquisition alternative, the dual energy approaches decompose the attenuation coefficients into components related to photoelectric absorption and Compton scattering. With the dual energy projection datasets, the nonlinear intensity measurements can be transformed to two simple linear integrals of the component coefficients, and the FBP can be used to reconstruct the unknown object. However, this technique requires two scans at different kVps and a sophisticated hardware setup, such as dual-energy X-ray tubes, rapid kVp switching, or energy discriminating detectors (either layered detectors or photo counting detectors).
In addition, high concentration iodinated contrast agent can also induce strong beam hardening artifacts. Some efforts have been made to account for the attenuation properties iodine, but with limited success. For example, an image-based beam hardening correction algorithm was developed to incorporate the attenuation properties of water, bone and iodine in terms of effective density. However, a pre-requisite of this technique is to accurately segment these three base materials into distinct regions. Also, a technique was developed to distinguish the three regions by measuring the voxel dynamics, but they used a series of threshold-based segmentation techniques, and the voxels containing low concentration or low dynamic iodinated contrast agent could be potentially mis-interpreted as soft tissue or bone minerals. Besides, both of the two techniques are limited to myocardial perfusion exam, as they only model the attenuation properties of blood-iodine mixture. Errors may arise in other perfusion exams, such as lung or breast perfusion exams.
In view of the foregoing, quantitative poly-energetic reconstruction schemes for X-ray imaging modalities is proposed to improve the performance of the X-ray imaging modalities.